Cranial perforators are special purpose drills which are used to bore holes through the skull during cranial surgery. Such holes may be needed to vent fluids from the region surrounding the brain, to provide small passageways to the brain for the insertion and removal of instruments, or to position a cranial saw for subsequent use in removing a larger piece of the skull.
Regardless of the end use of the hole being made, it is critical that the cranial perforator cease its boring action immediately after it passes through the skull and before it encounters, and thereby damages, the delicate dura tissue surrounding the brain, or the brain itself. To this end, cranial perforators have traditionally utilized a special "safety construction" designed to permit forward penetration by the perforator only so long as the perforator's leading tip is encountering hard bone, and to halt forward penetration by the perforator as soon as the perforator's leading tip passes through the hard bone and before it encounters the soft tissue beneath the bone. More particularly, cranial perforators have traditionally comprised a drill head assembly having a pair of drills disposed in concentric relation to one another, with the inner drill leading the outer drill so that a bore-counterbore opening is formed as the perforator penetrates into the skull. The two drills are coupled to a rear support and drive assembly via a special clutch arrangement such that both drills are enabled so long as the leading inner drill is encountering a resistive surface (i.e., bone) and both drills are disabled as soon as the inner drill stops encountering the resistive surface (i.e., as soon as it passes through the bone) while the outer drill is still encountering the resistive surface (i.e., while it is still cutting through the bone). Inasmuch as the leading inner drill and the trailing outer drill are adapted to cut in a bore-counterbore arrangement, the shoulder of bone formed at the intersection of the bore-counterbore opening automatically impedes further progress of the perforator toward the brain once the inner and outer drills are disabled. As a result, the surgeon using the cranial perforator does not have to concentrate entirely on the amount of pressure to be applied to the cranial perforator as the remaining bone becomes thinner and thinner, and generally need not fear that the perforator will penetrate too far into the head so as to damage the delicate dura tissue or the brain itself. Such cranial perforators have included both reusable and disposable models.
U.S. Pat. No. 4,600,006, issued July 15, 1986 to John W. Baker for "Cranial Perforator" discloses an improved form of cranial perforator incorporating the foregoing "safety construction". Other cranial perforators incorporating the foregoing "safety construction" are referenced in the specification of that patent or were cited in the prosecution of that patent. Still other cranial perforators incorporating the foregoing "safety construction" are disclosed in U.S. Pat. No. 4,803,982, issued Feb. 14, 1989 to John W. Baker for "Cranial Perforator".
Unfortunately, all of the "safety construction" cranial perforators developed to date are believed to suffer from a deficiency which can have catastrophic effects. This deficiency relates to the fact that all of the "safety construction" cranial perforators developed to date have their drill head assembly constructed so that the inner drill leads the outer drill and the inner and outer drills are adapted to cut in a bore-counterbore arrangement. Frequently, when drilling through very thin bone structures, the inner drill will pass all the way through the bone structure and into the delicate dura tissue, or even the brain, before the outer drill engages the bone structure, which engagement is necessary for the perforator's "safety construction" to impede further penetration of the perforator (i.e., by virtue of the fact that such engagement of the outer drill with the bone structure will (a) cause the inner and outer drills to be disengaged from the rear support and drive assembly and thereby drivingly disengaged, and (b) cause the outer drill, and hence the entire cranial perforator, to be supported on the shoulder of bone formed at the bore-counterbore opening in the skull). Thus, the perforator's "safety construction" becomes operative too late to prevent injury to the dura tissue or the brain. This situation occurs most often in pediatric drilling where the cranial bone is very thin, and in drilling in the thin rear temporal area of the skull in adults. Until now, a surgeon's skill and observations were his only tools to prevent penetration of the perforator into the dura or the brain in these cases.